U.S. patent application number 11/155850 was filed with the patent office on 2006-01-05 for polymer electrolyte membrane, membrane-electrode assembly, fuel cell system, and method for preparing the membrane-electrode assembly.
Invention is credited to Hee-Tak Kim, Ho-Jin Kweon.
Application Number | 20060003212 11/155850 |
Document ID | / |
Family ID | 35514333 |
Filed Date | 2006-01-05 |
United States Patent
Application |
20060003212 |
Kind Code |
A1 |
Kim; Hee-Tak ; et
al. |
January 5, 2006 |
Polymer electrolyte membrane, membrane-electrode assembly, fuel
cell system, and method for preparing the membrane-electrode
assembly
Abstract
A polymer electrolyte membrane for a fuel cell includes a proton
conductive polymer membrane and proton conductive microfibers
coated on either side of the proton conductive polymer
membrane.
Inventors: |
Kim; Hee-Tak; (Suwon-si,
KR) ; Kweon; Ho-Jin; (Suwon-si, KR) |
Correspondence
Address: |
CHRISTIE, PARKER & HALE, LLP
PO BOX 7068
PASADENA
CA
91109-7068
US
|
Family ID: |
35514333 |
Appl. No.: |
11/155850 |
Filed: |
June 16, 2005 |
Current U.S.
Class: |
429/480 ;
427/115; 429/483; 429/494; 429/524; 429/532; 429/535; 502/101 |
Current CPC
Class: |
H01M 4/921 20130101;
Y02P 70/50 20151101; H01M 8/1016 20130101; H01M 8/0245 20130101;
B82Y 30/00 20130101; H01M 2300/0082 20130101; H01M 8/0234 20130101;
H01M 2300/0094 20130101; Y02E 60/50 20130101; H01M 4/881
20130101 |
Class at
Publication: |
429/030 ;
429/033; 427/115; 502/101 |
International
Class: |
H01M 8/10 20060101
H01M008/10; B05D 5/12 20060101 B05D005/12; H01M 4/88 20060101
H01M004/88 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 30, 2004 |
KR |
10-2004-0050774 |
Claims
1. A polymer electrolyte membrane for a fuel cell, comprising: a
proton conductive polymer membrane; and proton conductive
microfibers coated on either side of the polymer membrane.
2. The polymer electrolyte membrane for the fuel cell according to
claim 1, wherein the proton conductive polymer membrane comprises a
material selected from the group consisting of perfluoro-based
polymers, benzimidazole-based polymers, polyimide-based polymers,
polyetherimide-based polymers, polyphenylenesulfide-based polymers,
polysulfone-based polymers, polyethersulfone-based polymers,
polyetherketone-based polymers, polyether-etherketone-based
polymers, polyphenylquinoxaline-based polymers, and combinations
thereof.
3. The polymer electrolyte membrane for the fuel cell according to
claim 1, wherein the proton conductive polymer membrane comprises a
proton conductive polymer selected from the group consisting of
poly(perfluorosulfonic acid), poly(perfluorocarboxylic acid), and
copolymers of fluorovinylether and tetrafluoroethylene including
sulfonic acid groups, defluorinated polyetherketone sulfide,
arylketones, poly(2,2'-(m-phenylene)-5,5'-bibenzimidazole),
poly(2,5-benzimidazole), and combinations thereof.
4. The polymer electrolyte membrane for the fuel cell according to
claim 1, wherein an average diameter of the proton conductive
microfibers is from 0.01 to 5 .mu.m.
5. The polymer electrolyte membrane for the fuel cell according to
claim 1, wherein the proton conductive microfibers are coated by an
electrospinning method.
6. The polymer electrolyte membrane for the fuel cell according to
claim 1, wherein the proton conductive microfibers comprise a
material selected from the group consisting of perfluoro-based
polymers, benzimidazole-based polymers, polyimide-based polymers,
polyetherimide-based polymers, polyphenylenesulfide-based polymers,
polysulfone-based polymers, polyethersulfone-based polymers,
polyetherketone-based polymers, polyether-etherketone-based
polymers, polyphenylquinoxaline-based polymers, and combinations
thereof.
7. The polymer electrolyte membrane for the fuel cell according to
claim 1, wherein the proton conductive microfibers comprise a
proton conductive polymer selected from the group consisting of
poly(perfluorosulfonic acid), poly(perfluorocarboxylic acid),
copolymers of fluorovinylether and tetrafluoroethylene including
sulfonic acid groups, defluorinated polyetherketone sulfide,
arylketones, poly(2,2'-(m-phenylene)-5,5'-bibenzimidazole),
poly(2,5-benzimidazole), and combinations thereof.
8. A membrane-electrode assembly comprising: a polymer electrolyte
membrane for a fuel cell having a proton conductive polymer
membrane and proton conductive microfibers coated on either side of
the polymer membrane; a catalyst layer coated on either side of the
polymer electrolyte membrane; and a gas diffusion layer positioned
on the catalyst layer.
9. The membrane-electrode assembly according to claim 8, wherein
the catalyst layer comprises a catalyst provided in an amount from
0.001 to 0.5 mg/cm.sup.2.
10. The membrane-electrode assembly according to claim 8, wherein
the catalyst layer comprises a catalyst with a specific surface
area between 10 and 500 m.sup.2/g.
11. The membrane-electrode assembly according to claim 8, wherein
the catalyst layer comprises a material selected from the group
consisting of platinum, ruthenium, osmium, platinum-ruthenium
alloys, platinum-osmium alloys, platinum-palladium alloys, and
platinum-M alloys, where M is a transition metal selected from the
group consisting of Ga, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn, and
combinations thereof.
12. The membrane-electrode assembly according to claim 8, wherein
the gas diffusion layer comprises a material selected from the
group consisting of carbon paper and carbon cloth.
13. The membrane-electrode assembly according to claim 8, further
comprising a microporous layer (MPL) between the catalyst layer and
the gas diffusion layer.
14. The membrane-electrode assembly according to claim 13, wherein
the MPL comprises a material selected from the group consisting of
graphite, carbon nanotube (CNT), fullerene (C60), activated carbon,
carbon black, and combinations thereof.
15. A fuel cell system, comprising: an electricity generating unit
including separators and a membrane-electrode assembly between the
separators, the membrane-electrode assembly including an anode and
a cathode and a polymer electrolyte membrane between the anode and
the cathode; a fuel supply unit for supplying fuel to the
electricity generating unit; and an oxidizing agent supply unit for
supplying oxidizing agent to the electricity generating unit,
wherein the polymer electrolyte membrane comprises a proton
conductive polymer membrane, and proton conductive microfibers
coated on either side of the polymer membrane.
16. A method for preparing a membrane-electrode assembly
comprising: coating a proton conductive polymer membrane with
proton conductive microfibers to prepare a polymer electrolyte
membrane for a fuel cell; depositing a catalyst on either side of
the polymer electrolyte membrane to form a catalyst layer; and
positioning a gas diffusion layer on the catalyst layer.
17. The method for preparing a membrane-electrode assembly
according to claim 16, wherein the proton conductive polymer
membrane comprises a material selected from the group consisting of
perfluoro-based polymers, benzimidazole-based polymers,
polyimide-based polymers, polyetherimide-based polymers,
polyphenylenesulfide-based polymers, polysulfone-based polymers,
polyethersulfone-based polymers, polyetherketone-based polymers,
polyether-etherketone-based polymers, polyphenylquinoxaline-based
polymers, and combinations thereof.
18. The method for preparing a membrane-electrode assembly
according to claim 16, wherein the proton conductive polymer
membrane comprises a proton conductive polymer selected from the
group consisting of poly(perfluorosulfonic acid),
poly(perfluorocarboxylic acid), copolymers of fluorovinylether and
tetrafluoroethylene including sulfonic acid groups, defluorinated
polyetherketone sulfide, arylketones,
poly(2,2'-(m-phenylene)-5,5'-bibenzimidazole),
poly(2,5-benzimidazole), and combinations thereof.
19. The method for preparing a membrane-electrode assembly
according to claim 16, wherein the proton conductive microfibers
comprise a material selected from the group consisting of
perfluoro-based polymers, benzimidazole-based polymers,
polyimide-based polymers, polyetherimide-based polymers,
polyphenylenesulfide-based polymers, polysulfone-based polymers,
polyethersulfone-based polymers, polyetherketone-based polymers,
polyether-etherketone-based polymers, polyphenylquinoxaline-based
polymers, and combinations thereof.
20. The method for preparing a membrane-electrode assembly
according to claim 16, wherein the proton conductive microfibers
comprise a proton conductive polymer selected from the group
consisting of poly(perfluorosulfonic acid),
poly(perfluorocarboxylic acid), copolymers of fluorovinylether and
tetrafluoroethylene including sulfonic acid groups, defluorinated
polyetherketone sulfide, arylketones,
poly(2,2'-(m-phenylene)-5,5'-bibenzimidazole),
poly(2,5-benzimidazole), and combinations thereof.
21. The method for preparing a membrane-electrode assembly
according to-claim 16, wherein the coating of the proton conductive
polymer membrane with the proton conductive microfibers is
performed by an electrospinning method.
22. The method for preparing a membrane-electrode assembly
according to claim 16, wherein the catalyst layer is formed using a
method selected from sputtering, thermal chemical vapor deposition
(CVD), plasma enhanced CVD (PECVD), thermal evaporation,
electrochemical deposition, e-beam evaporation, and combinations
thereof.
23. The method for preparing a membrane-electrode assembly
according to claim 16, wherein the catalyst is provided an amount
between 0.001 and 0.5 mg/cm.sup.2.
24. The method for preparing a membrane-electrode assembly
according to claim 16, wherein the catalyst layer comprises a
material selected from the group consisting of platinum, ruthenium,
osmium, platinum-ruthenium alloys, platinum-osmium alloys,
platinum-palladium alloys, platinum-M alloys, and combinations
thereof, wherein M is a transition metal selected from the group
consisting of Ga, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn.
25. The method for preparing a membrane-electrode assembly
according to claim 16, wherein the gas diffusion layer comprises a
material selected from the group consisting of carbon paper and
carbon cloth.
26. The method for preparing a membrane-electrode assembly
according to claim 16, further comprising forming a microporous
layer (MPL) between the catalyst layer and the gas diffusion
layer.
27. The method for preparing a membrane-electrode assembly
according to claim 26, wherein the MPL comprises a material
selected from the group consisting of graphite, carbon nanotube
(CNT), fullerene (C60), activated carbon, carbon black, and
combinations thereof.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to and the benefit of
Korean Patent Application No. 10-2004-0050774 filed on Jun. 30,
2004 in the Korean Intellectual Property Office, the entire content
of which is incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to a polymer electrolyte
membrane, a membrane-electrode assembly, a fuel cell system, and a
method for preparing a membrane-electrode assembly, and, more
particularly, to a polymer electrolyte membrane for a fuel cell
having a wide surface area, a membrane-electrode assembly with good
efficiency, and a fuel cell system including the same, and a method
for preparing the membrane-electrode assembly.
BACKGROUND OF THE INVENTION
[0003] A fuel cell is an electric power generating system for
producing electrical energy through a chemical reaction between
oxygen and hydrogen included in a hydrocarbon-based material such
as methanol, ethanol, or natural gas.
[0004] A fuel cell can be classified into a phosphoric acid type, a
molten carbonate type, a solid oxide type, a polymer electrolyte
type, or an alkaline type, depending upon the kind of electrolyte
used. Although each of these different types of fuel cells operates
in accordance with the same basic principles, they may differ from
one another in the kind of fuel, the operating temperature, the
catalyst, and the electrolyte used.
[0005] Recently, polymer electrolyte membrane fuel cells (PEMFCs)
have been developed. They have power characteristics that are
superior to conventional fuel cells, as well as lower operating
temperatures, and faster start and response characteristics.
Because of this, PEMFCs can be applied to a wide array of fields,
such as for transportable electrical sources for automobiles,
distributed power sources for houses and public buildings, and
small electrical sources for electronic devices.
[0006] A PEMFC is essentially composed of a stack, a reformer, a
fuel tank, and a fuel pump. The stack forms a body of the PEMFC,
and the fuel pump provides fuel stored in the fuel tank to the
reformer. The reformer reforms the fuel to generate hydrogen gas
and supplies the hydrogen gas to the stack, where it is
electrochemically reacted with oxygen to generate electrical
energy.
[0007] Alternatively, a fuel cell may include a direct methanol
fuel cell (DMFC) in which liquid methanol fuel is directly
introduced to the stack. Unlike a PEMFC, a DMFC does not require a
reformer.
[0008] In a fuel cell system described above, the stack for
generating the electricity has a structure in which several unit
cells, each having a membrane-electrode assembly (MEA) and a
separator (also referred to as a bipolar plate), are stacked
adjacent one another. The MEA has a structure in which a polymer
electrolyte membrane is positioned and adhered between an anode
(also referred to as a fuel electrode or an oxidation electrode)
and a cathode (also referred to as an air electrode or a reduction
electrode).
[0009] The separators function both as passageways for supplying
the fuel and the oxygen necessary for a reaction of the fuel cell
to the anode and the cathode, as well as conductors for serially
connecting the anode and the cathode of each MEA. The
electrochemical oxidation reaction of the fuel occurs at the anode,
and the electrochemical reduction reaction of the oxygen occurs at
the cathode. Due to movement of the electrons generated by the
reactions, electricity, heat, and water are collectively
produced.
[0010] The anode and the cathode typically include a platinum
catalyst. However, since platinum is too expensive a noble metal to
be used in large quantities, platinum is typically supported on a
carbon layer to reduce the amount of the platinum used.
[0011] However, supporting the platinum catalyst on the carbon can
result in certain shortcomings, such as a thick catalyst layer, a
catalyst layer having limited platinum storage capabilities, or a
deterioration of a fuel cell due to a bad contact condition between
a catalyst layer and an electrolyte membrane.
[0012] Therefore, it is desirable to develop an MEA that has a good
cell capacity even when the amount of the catalyst included in the
catalyst layer of the MEA is reduced.
SUMMARY OF THE INVENTION
[0013] An embodiment of the present invention provides a polymer
electrolyte membrane for a fuel cell with a high specific surface
area.
[0014] An embodiment of the present invention provides a
membrane-electrode assembly (MEA) in which the specific surface
area of a catalyst is high.
[0015] An embodiment of the present invention provides a fuel cell
system including the polymer electrolyte membrane with the high
specific surface area.
[0016] An embodiment of the present invention provides a method for
preparing the MEA in which the specific surface area of the
catalyst is high.
[0017] One embodiment of the present invention provides a polymer
electrolyte membrane for a fuel cell including a proton conductive
polymer membrane and proton conductive microfibers coated on either
side of the polymer membrane.
[0018] One embodiment of the present invention also provides a
membrane-electrode assembly including a polymer electrolyte
membrane for a fuel cell having a proton conductive polymer
membrane and proton conductive microfibers coated on either side of
the polymer membrane, a catalyst layer coated on either side of the
polymer electrolyte membrane, and a gas diffusion layer positioned
on the catalyst layer.
[0019] One embodiment of the present invention provides a fuel cell
system. The fuel cell includes an electricity generating unit
including separators and a membrane-electrode assembly between the
separators, the membrane-electrode assembly including an anode and
a cathode and a polymer electrolyte membrane between the anode and
the cathode; a fuel supplying unit for supplying fuel including
hydrogen or methanol to the electricity generating unit; and an
oxidizing agent supplying unit for supplying oxidizing agent to the
electricity generating unit. In this embodiment, the polymer
electrolyte membrane includes a proton conductive polymer membrane,
and a proton conductive microfiber coated on either side of the
polymer membrane.
[0020] One embodiment of the present invention provides a method
for preparing a membrane-electrode assembly. The method includes a
process in which a proton conductive polymer membrane is coated
with proton conductive microfibers to prepare a polymer electrolyte
membrane for a fuel cell; a catalyst is deposited on either side of
the polymer electrolyte membrane for the fuel cell to form a
catalyst layer; and a gas diffusion layer is positioned on the
catalyst layer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a plan view-schematically showing one side of a
polymer electrolyte membrane for a fuel cell according to the
present invention;
[0022] FIG. 2 is a cross sectional view schematically showing a
membrane-electrode assembly (MEA) according to the present
invention;
[0023] FIG. 3 is a cross sectional view schematically showing an
MEA according to the present invention further including a
microporous layer;
[0024] FIG. 4 is a schematic diagram illustrating a fuel cell
system in accordance with the present invention;
[0025] FIG. 5 is an exploded perspective view schematically showing
a electricity generating unit including a polymer electrolyte
membrane for a fuel cell according to the present invention;
and
[0026] FIG. 6 is a scanning electron microscope photograph of a
polymer electrolyte membrane according to Example 1.
DETAILED DESCRIPTION
[0027] FIG. 1 is a plan view schematically showing one side of a
polymer electrolyte membrane 100 for a fuel cell according to the
present invention. Referring to FIG. 1, the polymer electrolyte
membrane 100 for the fuel cell according to the present invention
includes a proton conductive polymer membrane 101 and proton
conductive microfibers 102 coated on either side of the proton
conductive polymer membrane 101.
[0028] In one embodiment, the proton conductive polymer membrane
101 includes a proton conductive polymer such as is typically used
as an electrolyte membrane material for a fuel cell. Exemplary
materials for the proton conductive polymer membrane 101 include
perfluoro-based polymers, benzimidazole-based polymers,
polyimide-based polymers, polyetherimide-based polymers,
polyphenylenesulfide-based polymers, polysulfone-based polymers,
polyethersulfone-based polymers, polyetherketone-based polymers,
polyether-etherketone-based polymers, polyphenylquinoxaline-based
polymers, and combinations thereof. Preferred proton conductive
polymers include poly(perfluorosulfonic acid),
poly(perfluorocarboxylic acid), copolymers of fluorovinylether and
tetrafluoroethylene including sulfonic acid groups, defluorinated
polyetherketone sulfide, arylketones,
poly(2,2'-(m-phenylene)-5,5'-bibenzimidazole),
poly(2,5-benzimidazole), and combinations thereof. However, the
proton conductive polymer membrane 101 of FIG. 1 included in the
polymer electrolyte membrane 100 according to the present invention
is not limited thereto.
[0029] In one embodiment, the average diameter of a proton
conductive microfiber 102 which is coated on either side of the
proton conductive polymer membrane 101 is from 0.01 to 5 .mu.m, and
more preferably from 0.01 to 0.5 .mu.m. When the average diameter
of the microfiber is less than 0.01 .mu.m, the process for
preparing the microfibers 102 becomes difficult due to the high
voltage needed for preparing them. Alternatively, when the average
diameter is more than 5 .mu.m, the increase of the surface area is
not sufficient.
[0030] In one embodiment, the proton conductive microfibers 102 are
coated on either side of the proton conductive polymer membrane 101
by an electrospinning method in which a polymer is spun by applying
a potential difference to a polymer melt or a polymer solution.
[0031] In one embodiment, the proton conductive microfibers 102
includes a proton conductive polymer such as is typically used as
an electrolyte membrane material for a fuel cell. Exemplary
materials for the proton conductive microfibers 102 include
perfluoro-based polymers, benzimidazole-based polymers,
polyimide-based polymers, polyetherimide-based polymers,
polyphenylenesulfide-based polymers, polysulfone-based polymers,
polyethersulfone-based polymers, polyetherketone-based polymers,
polyether-etherketone-based polymers, polyphenylquinoxaline-based
polymers, and combinations thereof. Preferred proton conductive
polymers include poly(perfluorosulfonic acid),
poly(perfluorocarboxylic acid), copolymers of fluorovinylether and
tetrafluoroethylene including sulfonic acid groups, defluorinated
polyetherketone sulfide, arylketones,
poly(2,2'-(m-phenylene)-5,5'-bibenzimidazole),
poly(2,5-benzimidazole), and combinations thereof. However, the
proton conductive microfibers 102 included in the polymer
electrolyte membrane 100 for the fuel cell according to the present
invention are not limited thereto.
[0032] In view of FIG. 1, since the surface area of either side of
the polymer electrolyte membrane 100 is large due to the proton
conductive microfibers 102, the area capable of contacting with a
catalyst layer of a membrane-electrode assembly (MEA) is also
increased, thereby improving the efficiency of the fuel cell.
[0033] FIG. 2 is a cross sectional view schematically showing an
MEA 10 according to the present invention. Referring to FIG. 2, the
MEA 10 includes the polymer electrolyte membrane 100 for the fuel
cell of FIG. 1. In addition, the MEA 10 includes catalyst layers
110, 110' coated on either side of the polymer electrolyte membrane
100 using a process such as deposition, and gas diffusion layers
120, 120' respectively positioned on the catalyst layers 110,
110'.
[0034] In one embodiment of the present invention, the content of
the catalyst in at least one of the catalyst layers 110, 110' is
from 0.001 to 0.5 mg/cm.sup.2, more preferably, from 0.01 to 0.05
mg/cm.sup.2. When the content of the catalyst in the catalyst
layers 110, 110' is less than 0.001 mg/cm.sup.2, output power of
the fuel cell is not sufficient. Alternatively, when the content of
the catalyst in the catalyst layers 110, 110' is more than 0.5
mg/cm.sup.2, optimal utilization of the catalyst may be
reduced.
[0035] Furthermore, in one embodiment, the specific surface area of
the catalyst included in the catalyst layers 110, 110' is in the
range of from 10 to 500 m.sup.2/g. Since the oxidation/reduction
reaction of the fuel cell occurs at the surface of the catalyst,
the larger the specific surface area, the better the efficiency of
the fuel cell. Therefore, when the specific surface area of the
catalyst is less than 10 m.sup.2/g, the efficiency of the fuel cell
is reduced. However, when it is more than 500 m.sup.2/g, the
process for preparing it becomes difficult.
[0036] In one embodiment, the catalyst layers 110, 110' includes a
catalyst selected from the group consisting of platinum, ruthenium,
osmium, platinum-ruthenium alloys, platinum-osmium alloys,
platinum-palladium alloys, and platinum-M alloys (where M is at
least one transition metal selected from the group consisting of
Ga, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) and combinations
thereof. Preferred catalysts include platinum, ruthenium, osmium,
platinum-ruthenium alloys, platinum-osmium alloys,
platinum-palladium alloys, platinum-cobalt alloys, platinum-nickel
alloys, and combinations thereof.
[0037] As shown in FIG. 2, the gas diffusion layers (GDL) 120, 120'
are respectively positioned on the catalyst layers 110,110'. The
gas diffusion layers 120, 120' are for supplying hydrogen gas and
oxygen gas to the respective catalyst layers 110, 110', to help
form a three-phase interface of catalyst-electrolyte membrane-gas.
In one embodiment, at least one of the gas diffusion layers 120,
120' is made of a carbon paper or a carbon cloth.
[0038] A microporous layer (MPL) may be further included between
the catalyst layers 110, 110' and the gas diffusion layers 120,
120', to help diffusion of the hydrogen gas and the oxygen gas.
[0039] FIG. 3 is a cross-sectional view schematically showing a
membrane-electrode assembly (MEA) 10' according to such an
embodiment of the present invention. Similar to the previous
embodiment, MEA 10' includes a polymer electrolyte membrane 100,
catalyst layers 210, 210' and gas diffusion layers 220, 220' with
the difference being which the inclusion of microporous layers
(MPLs) 221, 221' between the catalyst layers 210, 210' and gas
diffusion layers 220, 220'.
[0040] In one embodiment, each of the microporous layers 221, 221'
is a carbon layer in which micropores of a few .mu.m or less are
formed, and preferably, it comprises a material selected from the
group consisting of graphite, carbon nanotube (CNT), fullerene
(C60), activated carbon, carbon black, and combinations
thereof.
[0041] The catalyst of the catalyst layers of either embodiment may
be coated by direct deposition of the catalyst, and such catalyst
layers have excellent performance due to their large surface
areas.
[0042] FIG. 4 is a schematic diagram illustrating a fuel cell
system in accordance with the present invention, and FIG. 5 is an
exploded perspective view schematically showing a electricity
generating unit 1 including a polymer electrolyte membrane (e.g.,
the polymer electrolyte membrane 100 of FIG. 1) according to the
present invention.
[0043] Referring to FIGS. 4 and 5, the fuel cell system of the
present invention includes a fuel supply unit 2 for supplying fuel
including hydrogen, an oxidizing agent supply unit 3 for supplying
oxidizing agent (such as oxygen contained in air), and an
electricity generating unit 1 for generating electricity by
performing electrochemical reactions of the fuel and the oxidizing
agent.
[0044] Furthermore, the electricity generating unit 1 of the
present invention includes at least one unit cell 30 which includes
the membrane-electrode assembly 10 including the polymer
electrolyte membrane 100, and an anode and a cathode positioned on
either side of the polymer electrolyte membrane 100, and separators
20. The membrane-electrode assembly 10 is between the separators
20, and the polymer electrolyte membrane 100 of the
membrane-electrode assembly 10 includes the proton conductive
polymer membrane 101, and the proton conductive microfibers 102
coated on either side of the proton conductive polymer membrane
101.
[0045] A method for preparing a membrane-electrode assembly
according to the present invention includes: a process in which
proton conductive microfibers are coated on either side of a proton
conductive polymer membrane to prepare a polymer electrolyte
membrane for a fuel cell; a process in which catalyst is deposited
on either side of the above-mentioned polymer electrolyte membrane
to form a catalyst layer; and a process in which a gas diffusion
layer is positioned on the catalyst layer.
[0046] In one embodiment, the proton conductive polymer membrane
used for preparing the membrane-electrode assembly includes a
proton conductive polymer such as is typically used as an
electrolyte membrane material for a fuel cell. Examples of such
proton conductive polymers include perfluoro-based polymers,
benzimidazole-based polymers, polyimide-based polymers,
polyetherimide-based polymers, polyphenylenesulfide-based polymers,
polysulfone-based polymers, polyethersulfone-based polymers,
polyetherketone-based polymers, polyether-etherketone-based
polymers, polyphenylquinoxaline-based polymers, and combinations
thereof. Preferred proton conductive polymers include
poly(perfluorosulfonic acid), poly(perfluorocarboxylic acid),
copolymers of fluorovinylether and tetrafluoroethylene including
sulfonic acid groups, defluorinated polyetherketone sulfide,
arylketones, poly(2,2'-(m-phenylene)-5,5'-bibenzimidazole),
poly(2,5-benzimidazole), and combinations thereof. However, the
proton conductive polymer membrane included in the polymer
electrolyte membrane for the fuel cell according to the present
invention is not limited thereto.
[0047] In one embodiment, the proton conductive microfibers used
for preparing the above-mentioned membrane-electrode assembly
include a proton conductive polymer. Exemplary materials for the
proton conductive microfiber include perfluoro-based polymers,
benzimidazole-based polymers, polyimide-based polymers,
polyetherimide-based polymers, polyphenylenesulfide-based polymers,
polysulfone-based polymers, polyethersulfone-based polymers,
polyetherketone-based polymers, polyether-etherketone-based
polymers, polyphenylquinoxaline-based polymers, and combinations
thereof. Preferred proton conductive polymers include
poly(perfluorosulfonic acid), poly(perfluorocarboxylic acid),
copolymers of fluorovinylether and tetrafluoroethylene including
sulfonic acid groups, defluorinated polyetherketone sulfide,
arylketones, poly(2,2'-(m-phenylene)-5,5'-bibenzimidazole),
poly(2,5-benzimidazole), and combinations thereof. However, the
materials for the proton conductive microfibers used for preparing
the membrane-electrode assembly according to the present invention
are not limited thereto.
[0048] The above-mentioned proton conductive microfibers may be
coated by an electrospinning method on either side of the proton
conductive polymer membrane. The electrospinning method is a
technology in which a large potential differential is applied
between a grounded collecting screen and a proton conductive
polymer melt or a proton conductive polymer solution. A non-woven
mat is prepared by the electrospinning method, and the microfibers
produced by such method have a very large surface area. In more
detail, an electrospinning method can be achieved in accordance
with an article in, Applied Chemistry, Vol. 2, No. 2, 1998, which
is incorporated by reference herein in its entirety. However, the
scope of the present invention is not limited by the above
described electrospinning method, and those skilled in the art
would recognize that the method may be varied by other suitable
methods.
[0049] In one embodiment of the present invention, the voltage
applied to the proton conductive polymer melt or the proton
conductive polymer solution is from 1 to 1000 kV, and preferably
from 5 to 25 kV.
[0050] Catalyst is coated by deposition on either side of the
polymer electrolyte membrane to form the catalyst layer. In one
embodiment, the content of the catalyst included in the catalyst
layer is from 0.001 to 0.5 mg/cm.sup.2, and preferably from 0.01 to
0.05 mg/cm.sup.2.
[0051] Suitable deposition methods for depositing the catalyst
include sputtering, thermal chemical vapor deposition (CVD), plasma
enhanced CVD (PECVD), thermal evaporation, electrochemical
deposition, and e-beam evaporation methods.
[0052] In one embodiment, the catalyst layer includes a material
selected from the group consisting of platinum, ruthenium, osmium,
platinum-ruthenium alloys, platinum-osmium alloys,
platinum-palladium alloys, platinum-M alloys (where M is at least
one kind of transition metal selected from the group consisting of
Ga, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn), and combinations
thereof. Preferred catalysts include platinum, ruthenium, osmium,
platinum-ruthenium alloys, platinum-osmium alloys,
platinum-palladium alloys, platinum-cobalt alloys, platinum-nickel
alloys, and combinations thereof.
[0053] The gas diffusion layer is positioned on the catalyst layer.
The gas diffusion layer plays a role in vigorously supplying
hydrogen gas and/or oxygen gas to the catalyst layer, to help in
forming a three-phase interface of the catalyst-electrolyte
membrane-gas, and in one embodiment is made of a carbon paper or a
carbon cloth.
[0054] A microporous layer may be additionally positioned between
the catalyst layer and the gas diffusion layer to help diffusion of
the hydrogen gas and/or the oxygen gas.
[0055] In one embodiment, the microporous layer is a carbon layer
in which micropores are formed, and preferably includes a material
selected from the group consisting of graphite, carbon nanotube
(CNT), fullerene (C60), activated carbon, carbon black, and
combinations thereof.
[0056] In view of the foregoing, a membrane-electrode assembly of
the present invention includes a catalyst layer coated by direct
deposition of catalyst, and has a catalyst layer with excellent
performance due to its large surface area, thereby improving the
performance of the membrane-electrode assembly.
[0057] The following examples further illustrate the present
invention in more detail, but the present invention is not limited
by these examples.
EXAMPLE 1
Preparation of a Polymer Electrolyte Membrane
[0058] After a poly(perfluorosulfonic acid) membrane (DuPont Co.
Nafion.RTM. 112) was set on a grounded collecting screen in a
chamber having a nozzle, a poly(perfluorosulfonic acid) solution
(DuPont Co. Nafion.RTM. solution) was put into the nozzle, and a
voltage of 50 kV was applied to the solution. While the solution
was released from the nozzle by the potential difference,
poly(perfluorosulfonic acid) microfibers having an average diameter
of 0.1 .mu.m were coated on one side of the poly(perfluorosulfonic
acid) membrane (Nafion.RTM. 112, Dupont).
[0059] By the same process, poly(perfluorosulfonic acid)
microfibers were coated on the other side of the
poly(perfluorosulfonic acid) membrane (Nafion.RTM. 112, Dupont) to
prepare a polymer electrolyte membrane. FIG. 6 is a scanning
electron microscope photograph of a surface of the polymer
electrolyte membrane prepared according to above-mentioned
process.
EXAMPLE 2
Preparation of a Membrane-Electrode Assembly
[0060] Platinum was deposited by sputtering on either side of the
polymer electrolyte membrane prepared according to Example 1 to
form a catalyst layer having 0.04 mg/cm.sup.2 of platinum.
[0061] Moreover, a carbon cloth, covered with a microporous layer
consisting of activated carbon, was positioned on each catalyst
layer to prepare a membrane-electrode assembly.
EXAMPLE 3
Preparation of a Fuel Cell
[0062] Separators were positioned on either side of the
membrane-electrode assembly prepared according to Example 2 to
prepare a fuel cell.
COMPARATIVE EXAMPLE 1
Preparation of a Membrane-Electrode Assembly
[0063] Platinum was deposited by sputtering on either side of a
polymer electrolyte membrane to form a catalyst layer having 0.04
mg/cm.sup.2 of platinum without coating a microfiber on the
poly(perfluorosulfonic acid) membrane (Nafion.RTM. 112). The
poly(perfluorosulfonic acid) membrane on which platinum was
deposited was positioned between two sheets of carbon cloth covered
with a microporous layer consisting of an activated carbon
substantially in the same manner as in Example 2, to prepare a
membrane-electrode assembly.
COMPARATIVE EXAMPLE 2
Preparation of a Fuel Cell
[0064] Separators were stacked on either side of the
membrane-electrode assembly prepared according to Comparative
Example 1 to produce a fuel cell.
[0065] Table. 1 shows the specific surface area of the catalyst
layers in the preparation process of the membrane-electrode
assemblies according to Example 2 and Comparative Example 1.
TABLE-US-00001 TABLE 1 Comparative Example 2 Example 1 Specific
surface area of catalyst layer 25 3 (m.sup.2/g)
[0066] As shown in Table 1, the specific surface area of the
catalyst layer in the membrane-electrode assembly prepared
according to Example 2 is higher by a factor of more than 8 as
compared to Comparative Example 1.
[0067] Table 2 shows current densities estimated at 60.degree. C.,
after vapor-saturated oxygen and hydrogen gases were injected
respectively to the cathodes and anodes of fuel cells prepared
according to Example 3 and Comparative Example 2. TABLE-US-00002
TABLE 2 Example 3 Comparative Example 2 Current density(A/cm.sup.2)
at 0.6 V 1.3 0.2
[0068] As shown in Table 2, the current density of the fuel cell in
Example 2, including the membrane-electrode assembly according to
the present invention, is 6 times higher than that of Comparative
Example 1.
[0069] In view of the foregoing, since catalyst is coated by
deposition directly on either of a polymer electrolyte membrane for
a fuel cell with a high specific surface area in a
membrane-electrode assembly according to the present invention, the
specific surface area of the catalyst is high, and consequently the
capability of a fuel cell is improved.
[0070] While the invention has been described in connection with
certain exemplary embodiments, it is to be understood by those
skilled in the art that the invention is not limited to the
disclosed embodiments, but, on the contrary, is intended to cover
various modifications included within the spirit and scope of the
appended claims and equivalents thereof.
* * * * *